My Lab wont retrieve

Question
My yellow lab is one year old, very sweet and gentle, never been hyper.  She has been raised with an australian shepherd (she's very high strung) since they both were 6 weeks old.  The aussie loves to fetch and the lab never would.  It just sits and watches or sometime tries but the aussie always bets her to it.  Even when I've tried with the aussie locked up the lab will not fetch.  Any ideas?

Answer
Let her see the Aussie getting treats for fetching.  Make a great fuss over the ball or whatever.  Let her see you treat it as a high value item.  Then give her a chance alone with it, having treats ready.  A year ago I was having some trouble with a Golden.  As long as I had treats, he wouldn't go after the stick.  Sometimes treats don't work.